1. Field of the Invention
The present invention relates to a Transcutaneous Electric Nerve Stimulator (TENS) electrode that comprises a conductive layer containing both an inner perimeter(s) and an outer perimeter(s).
2. Description of the Related Art
The conscious perception of pain involves the transmission of sensory information across three primary types of neurons that form a pathway from the body to the brain. Responding directly to the painful stimulus, peripheral afferent fibers transmit a message indicating the presence of pain to intermediate neurons in the dorsal horn of the spinal cord. Acting as middlemen, intermediate neurons in the dorsal horn receive the message of pain from peripheral fibers and relay it to spinal cord neurons projecting to the brain, where the pain is consciously perceived.
Two types of peripheral afferent fibers respond directly to painful stimuli. Fast conducting A fibers respond to the immediate presence of pain, such as a prick or a burn. The activation of A fibers generally elicits a withdraw reflex. Slow conducting C fibers are responsible for the perception of persistent, sustained, lingering pain. Unlike A fibers that respond fast and turn off quickly, C fibers are slow to respond and continue to respond after the painful stimulus is removed.
The gate control theory of Mazack, R. & Wall, P. D. (1965) Pain mechanisms: a new theory. Science, 150, 971-979, proposes that dorsal horn neurons act as a gate increasing or decreasing the transmission of nerve impulses from peripheral fibers to spinal cord neurons projecting to the brain. Decreasing the responsiveness of dorsal horn neurons to C fiber stimulation will retard or prevent dorsal horn neurons from passing on the message of pain perception from C fibers to spinal neurons projecting to the brain. This will prevent the brain from becoming aware of the presence of persistent, sustained, lingering pain thereby providing an analgesic effect against persistent, sustained, lingering pain.
Reducing a neuron's responsiveness to stimulation can be accomplished by the induction of long term depression (LTD). LTD is a cellular phenomenon in which a neuron's sensitivity to incoming stimulation decreases thereby requiring an increased level of stimulation to activate the neuron. Activating A fiber afferents leading to the dorsal horn with low frequency, low intensity stimulation induces LTD in dorsal horn neurons to C fiber stimulation. (Liu, X. G., Morton, C. R., Azkue, J. J., Zimmermann, M., & Sandkühler, J. (1998) Long-term depression of C-fibre-evoked spinal field potentials by stimulation of primary afferent Aδ-fibres in the adult rat. European Journal of Neuroscience, 10, 3069-3075.) Activating A fiber afferents leading to the dorsal horn with high frequency, low intensity stimulation produces a more reliable and stable LTD in dorsal horn neurons to C fiber stimulation. (Ikeda, H., Asai, T., & Murase, K. (2000) Robust changes of afferent-induced excitation in the rat spinal dorsal horn after conditioning high-frequency stimulation. Journal of Neurophysiology, 83, 2412-2420 and Liu, X. G., Morton, C. R., Azkue, J. J., Zimmermann, M., & Sandkühler, J. (1998) Long-term depression of C-fibre-evoked spinal field potentials by stimulation of primary afferent Aδ-fibres in the adult rat. European Journal of Neuroscience, 10, 3069-3075.)
High and low frequency, low intensity stimulation that activates A fiber afferents stimulates the release of endogenous opiates that are critical for the induction of LTD in dorsal horn neurons to C fiber stimulation. Administering low doses of Nalaxone, an opiate receptor blocker, inhibits low frequency, low intensity stimulation induced LTD, but not high frequency, low intensity stimulation induced LTD. (Ikeda, H., Asai, T., & Murase, K. (2000) Robust changes of afferent-induced excitation in the rat spinal dorsal horn after conditioning high-frequency stimulation. Journal of Neurophysiology, 83, 2412-2420.) High dose Nalaxone inhibits high frequency, low intensity stimulation induced LTD. (Ikeda, H., Asai, T., & Murase, K. (2000) Robust changes of afferent-induced excitation in the rat spinal dorsal horn after conditioning high-frequency stimulation. Journal of Neurophysiology, 83, 2412-2420.) These findings imply that the release of opiates is critical for the induction of LTD in dorsal horn neurons and that the level of opiate release is positively correlated with the firing frequency of A fiber afferents.
The administration of opiates and opiate derivatives is considered the gold standard for managing persistent, lingering, sustained pain. However, the use of these compounds is severely limited by the development of tolerance and addiction in patients. Through continued use of opiates and opiate derivatives, patients develop a tolerance or loss of analgesic efficacy, requiring the administration of higher doses to produce the necessary level of analgesia. However, patients do not develop tolerances to the toxic effects of opiates and opiate derivatives, such as respiratory depression. Thus, continued use of opiates and opiate derivatives will eventually place the patient in a situation where the dose required to produce sufficient analgesia is lethal. Consequently, the prolonged use of opiates to manage pain is not possible. Therefore, there is a need for a pain management strategy that utilizes the efficacy of opiates and avoids the development of tolerance and addiction.
Within the above mentioned LTD paradigms, electrical stimulation was applied directly to the portion of the A fiber afferents located within the dorsal horn of the spinal chord. The rate of firing of stimulated A fiber afferents was controlled by the frequency of stimulation, with the higher the frequency of stimulation, the higher the rate of firing. The selective activation of A fiber afferents was controlled by the intensity of stimulation. Low intensity stimulation selectively activated A fiber afferents without activating C fiber afferents. The insertion of electrodes into the dorsal horn of patients suffering from persistent, sustained, lingering pain to stimulate a fast rate of A fiber afferent firing would be painful and impractical. Rather, noninvasive stimulation of A fiber afferents is preferred. TENS has been proven to be an effective, noninvasive treatment for the management of pain. TENS is believed to induce LTD in dorsal horn neurons to C-fiber stimulation by the activation of A fiber afferents. (Liu, X. G., Morton, C. R., Azkue, J. J., Zimmermann, M., & Sandkühler, J. (1998) Long-term depression of C-fibre-evoked spinal field potentials by stimulation of primary afferent Aδ-fibres in the adult rat. European Journal of Neuroscience, 10, 3069-3075.) The short lived analgesic effect induced by TENS treatment protocols and devices indicates that such protocols and devices induce low firing rates in A fiber afferents. Accordingly, there is a need for a TENS treatment protocol and device that induces a higher rate of firing in A fiber afferents to produce a longer lived analgesia in patients suffering from persistent, sustained, lingering pain.